In a fossil-fired steam generator the energy of a fossil fuel is used to generate superheated steam which can subsequently be supplied to a steam turbine for the purpose of generating electricity, in a power station for example. In particular at the steam temperatures and pressures typical in a power station environment, steam generators are generally implemented as water tube boilers, which is to say that the supplied water flows in a plurality of tubes which assimilate the energy in the form of radiant heat from the burner flames and/or through convection and/or through thermal conduction from the flue gas being produced during the combustion process.
In the region of the burners the steam generator tubes in this case typically form the combustion chamber wall in that they are welded to one another in a gas-tight arrangement. Steam generator tubes disposed in the flue gas duct can also be provided in other areas downstream of the combustion chamber on the flue gas side.
Fossil-fired steam generators can be categorized according to a multiplicity of criteria. For example, steam generators can be classified into vertical and horizontal design types, based on the flow direction of the gas flow. In the context of fossil-fired steam generators constructed in a vertical design a distinction is generally made in this case between one-pass and two-pass boilers.
In a single-pass or tower-type boiler the flue gas generated as a result of the combustion process in the combustion chamber always flows vertically from bottom to top. All the heating surfaces disposed in the flue gas duct are located on the flue gas side above the combustion chamber. Tower-type boilers offer a comparatively simple construction and simple containment of the stresses resulting due to the thermal expansion of the tubes. Furthermore, all the heating surfaces of the steam generator tubes disposed in the flue gas duct are horizontal and can therefore be drained of water completely, which can be desirable in environments exposed to risk of frost.
In the case of the two-pass boiler a horizontal gas duct is connected downstream in an upper region of the combustion chamber on the flue gas side, said horizontal gas duct leading into a vertical gas duct. In this second vertical gas duct the gas usually flows vertically from top to bottom. In the two-pass boiler there is therefore a multiple redirection of the flue gas. Advantages of this type of design are, for example, the lower overall height of the structure and the lower manufacturing costs resulting therefrom.
Furthermore, steam generators can be implemented as gravity circulation, forced circulation or once-through steam generators. In a once-through steam generator the heating of a plurality of evaporator tubes leads to a complete evaporation of the flow medium in the evaporator tubes in a single pass. Following its evaporation the flow medium—typically water—is supplied to superheater tubes connected downstream of the evaporator tubes and is superheated there. The position of the evaporation endpoint, i.e. the location at which the water component of the flow has totally evaporated, is in this case variable and dependent on operating mode. During full-load operation of a once-through steam generator of this type the evaporation endpoint is located for example in an end region of the evaporator tubes, such that the superheating of the evaporated flow medium commences already in the evaporator tubes (with the nomenclature used, this description is, strictly speaking, only valid for partial loads with subcritical pressure in the evaporator. For clarity of illustration purposes, however, this manner of presentation is used throughout in the following description).
In contrast to a gravity circulation or forced circulation steam generator, a once-through steam generator is not subject to any pressure limiting, which means that it can be dimensioned for live steam pressures far in excess of the critical pressure of water (PCri≈221 bar)—at which water and steam cannot occur simultaneously at any temperature and consequently also no phase separation is possible.
In low-load operation or during the startup phase a once-through steam generator of said type is usually operated at a minimum flow of flow medium in the evaporator tubes in order to ensure reliable cooling of the evaporator tubes. Toward that end, particularly at low loads of, for example, less than 40% of the design load, the pure once-through mass flow through the evaporator is usually no longer sufficient in itself to cool the evaporator tubes and for that reason an additional throughput of flow medium is superimposed in the course of the circulation on the once-through pass of flow medium through the evaporator. The minimum flow of flow medium in the evaporator tubes that is provided under normal operating conditions is consequently not completely evaporated in the evaporator tubes during the startup phase or in low-load operation, with the result that in an operating mode of said type unevaporated flow medium, in particular a water-steam mixture, is still present at the end of the evaporator tubes.
However, since the superheater tubes which are typically connected downstream of the evaporator tubes of the once-through steam generator only after the flow medium has passed through the combustion chamber walls are not designed for a throughflow of unevaporated flow medium, once-through steam generators are generally implemented in such a way that an ingress of water into the superheater tubes is reliably avoided also during the startup phase and in low-load operation. Toward that end the evaporator tubes are typically connected to the superheater tubes disposed downstream of them by way of a water separation system. In this arrangement the water separator effects a separation of the water-steam mixture emerging from the evaporator tubes during the startup phase or in low-load operation into water and steam. The steam is supplied to the superheater tubes connected downstream of the water separator, whereas the separated water is returned to the evaporator tubes via a circulating pump, for example, or can be discharged by way of a blow-down tank.
In this arrangement the water separation system can comprise a multiplicity of water separating elements which are directly integrated into the tubes. In this case a water separating element can be associated in particular with each of the evaporator tubes connected in parallel. Furthermore, the water separating elements can be embodied as what are called T-piece water separating elements. Here, each T-piece water separating element comprises an inflow tube section connected in each case to the upstream evaporator tube and, viewed in its longitudinal direction, transitions into a water discharge tube section, with an outflow tube section connected to the downstream superheater tube branching off in the transition zone.
By virtue of this type of design the T-piece water separating element is embodied for effecting an inertial separation of the water-steam mixture flowing out of the upstream evaporator tube and into the inflow tube section. Specifically, owing to its comparatively high level of inertia the water fraction of the flow medium flowing in the inflow tube section by preference continues to flow straight on past the transition point in an axial extension of the inflow tube section and consequently passes into the water discharge tube section and from there usually flows further into a connected collecting vessel. By contrast, the steam fraction of the water-steam mixture flowing in the inflow tube section is better able, by virtue of its comparatively low level of inertia, to follow an imposed redirection and consequently flows via the outflow tube section to the downstream superheater tube section. A once-through steam generator based on this type of design is known, for example, from EP 1 701 091.
In a once-through steam generator having a water separation system configured in such a way the decentralized integration of the water separation function into the individual tubes of the tube system of the once-through steam generator means that the water can be separated without prior collection of the flow medium flowing out of the evaporator tubes. It also means that the flow medium can be passed on directly into the downstream superheater tubes.
Owing to the manner of construction the transfer of flow medium to the superheater tubes is furthermore not restricted just to steam; rather, a water-steam mixture can now also be passed on to the superheater tubes through overfeeding of the water separating elements. By this means the evaporation endpoint can be shifted into the superheater tubes as necessary. This enables a particularly high level of operational flexibility to be achieved even during the startup phase or in the low-load mode of operation of the once-through steam generator. In particular the live steam temperature can be regulated within comparatively wide limits by controlling the feedwater volume.
With systems of this kind it is, however, necessary to take into account that because the water separation function is integrated into the individual tubes a comparatively high number of individual tube sections or elements is required specifically in the region of the separation system.